Fuel cells are highly expected as electricity generation systems, among which polymer solid electrolyte type fuel cells, which use polymer electrolytes as the electrolytes, have lower operation temperatures and are compact as compared to phosphoric acid type fuel cells and the like, accordingly are regarded as promising as power supplies for electric cars.
Now, a polymer solid electrolyte type fuel cell has a laminate structure comprising two electrodes, namely, a fuel electrode and an air electrode, and a polymer solid electrolyte film sandwiched between these electrodes; the fuel electrode is supplied with a hydrogen containing fuel and the air electrode is supplied with oxygen or air, and thus the electric power is generated by the oxidation and reduction reactions occurring in the respective electrodes. To these two electrodes, a mixture containing a catalyst for accelerating the chemical reactions and a solid electrolyte is generally applied. As the catalysts constituting the electrodes, widely used are platinum catalysts in which platinum, satisfactory in catalytic activity, is made to be supported.
Different characteristics are demanded for a catalyst for use in a polymer solid electrolyte type fuel cell; the demanded characteristics for the fuel electrode and those for the air electrode are probably different from each other. The catalyst for use in the fuel electrode is demanded to have the resistance to catalyst poisoning due to carbon monoxide, in addition to high catalytic activity. As the hydrogen supplied to the fuel electrode, the reformed hydrogen obtained from methanol or the like is regarded as promising; however, carbon monoxide is contained in the reformed hydrogen as an impurity, and a problem occurs that the carbon monoxide is adsorbed on the catalyst particles to deactivate the catalyst. This is the reason why the resistance to catalyst poisoning due to carbon monoxide is demanded. Accordingly, for the purpose of improving the resistance to catalyst poisoning due to carbon monoxide, supported ruthenium catalysts are widely applied as the catalyst for the fuel electrode, in addition to supported platinum catalysts.
In these years, however, the practical use of polymer solid electrolyte type fuel cells comes to be established, and in this context a new problem is confirmed to be involved in the catalyst for use in the fuel electrode. This problem is the one that the cell characteristics are degraded when the fuel becomes deficient during the fuel cell operation. More specifically, when some abnormal condition somehow occurs in fuel supply during the steady operation of a fuel cell, the activity of the catalyst in the fuel electrode is degraded owing to the fuel deficiency and the cell characteristics are degraded, causing trouble in steady supply of electric power.
Additionally, when such catalyst activity degradation due to fuel deficiency occurs, if the catalyst activity is recovered by the fuel supply that is once again made normal, the cessation of the electric power supply is temporal and nonfatal. However, according to the previous reports, the catalyst activity degradation caused by the fuel deficiency is irreversible in nature, and it has been confirmed that the catalyst activity is not fully recovered by once again supplying the fuel.
As a countermeasure against the irreversible deactivation of the catalyst caused by the fuel deficiency, the establishment of a system free from cessation of fuel supply can be said most important. However, even if such a peripheral system can be improved, on the assumption of a worse case scenario, it is also preferable to improve the catalyst of the fuel electrode and the fuel cell themselves so that the characteristics thereof may not be degraded when the fuel becomes deficient.
Now, as one of the remedies having hitherto been studied for the fuel cell catalyst, for example, the addition of ruthenium oxide (RuO2) or iridium oxide (IrO2) to the catalyst layer has been known. Additionally, it has been claimed to be effective that, as additional remedies for improvement, the adopted carrier is made to be a carrier stable in oxidation properties such as graphitized carbon, titanium oxide (Ti4O7) and the like, and moreover, the amount of the supported catalyst particles is increased (for details of these remedies for improvement, see the international publication gazettes, WO 01/1527, WO 01/152547).
However, according to the investigation conducted by the present inventors, these remedies can be effective to a certain extent but not necessarily to a sufficient extent, and yield non-negligible degradation in characteristics when the fuel is deficient. Accordingly, as for the catalyst for use in the fuel electrode, it is necessary to find additional improvements other than these remedies.
The present invention has been made with the background described above, and takes as its object the provision of a catalyst hardly degradable in catalytic activity even when the fuel deficiency occurs, as the catalyst for use in the fuel electrode of a polymer solid electrolyte type fuel cell.